Polarizers that are used in polarizing plates are optical elements for polarizing natural light or any polarized light in a certain direction, and are widely used in display devices such as liquid crystal display devices and organic light-emitting devices (OLED). Polarizers that are currently used in the display devices are generally made of a polyvinyl alcohol-based polarizing film which contains iodine or a dichroic dye and in which molecular chains are oriented in a specific direction.
The polyvinyl alcohol-based polarizing film is manufactured by dyeing a polyvinyl alcohol-based film with iodine or a dichroic dye, and then crosslinking the film and stretching the film in a specific direction. Herein, the stretching process may be performed either by wet stretching in a solution such as an aqueous boric acid solution or an aqueous iodine solution or by dry stretching in air, and the film is generally stretched at least five times. However, in this conventional manufacturing process, it is required that the thickness of the polyvinyl alcohol-based film before stretching should exceed 60 μm so that the film is stretched without breakage. If the thickness of the polyvinyl alcohol-based film before stretching is thinner than 60 μm, the side ends perpendicular to the stretching direction of the polyvinyl alcohol-based film will be easily rolled up due to an increase in the degree of swelling of the film, and the rolled portion adheres to the film surface to cause surface non-uniformity. In addition, due to the thin thickness, the modulus per unit area in the stretching process will increase, and thus breakage of the film will easily occur.
In recent years, as display devices have become thinner, polarizers have also been required to have a thinner thickness. However, when conventional polyvinyl alcohol-based films having a thickness before stretching of more than 60 μm are used, there is a limit to reducing the thickness of polarizers. Thus, studies have been attempted to manufacture thinner polarizers.
Specifically, the prior art discloses a method of manufacturing a thin polarizer using a laminate prepared either by coating a hydrophilic polymer layer on a substrate layer or by co-extruding a substrate layer-forming material and a hydrophilic polymer layer-forming material, rather than stretching a polyvinyl alcohol-based film alone. However, the coating or co-extrusion method had problems in that it is not easy to separate a polyvinyl alcohol layer from a substrate after stretching, and because a high peel force is required for this separation, the polyvinyl alcohol layer is likely to be damaged or deformed in the separation process, resulting in deterioration in the optical properties such as degree of polarization of the polyvinyl alcohol film. Further, because the coating or co-extrusion method is performed either by extruding polyvinyl alcohol resin after melting or by applying a coating solution of polyvinyl alcohol, the physical properties of the polyvinyl alcohol film manufactured are likely to change depending on extrusion conditions, coating conditions or film-forming conditions, and thus the physical properties of the resulting polyvinyl alcohol film may be deteriorated. In addition, there is a shortcoming in that the stretching property of the polyvinyl alcohol-based film may change depending on the properties of the substrate.